Question

I have Projelements, which are like:

• Milestone
• Task
• … etc …

Each Projelement can have one or more embedded Comments (thanks to Mongoid).

I want to be able to easily say:

Give me a list of Projelements sorted by when they were last updated

Where last_updated for a Projelement is the larger of:

• this_projelement.updated_at OR
• if this_projelement.comments.count > 0 then this_projelement.comments.last.updated_at

I can create a Projelement class method last_updated_at with the above logic.

But what I really want to end up with is doing a query of my Projelements, retrieving the last updated Projelement using the above logic, in the same way as you’d do a query using an attribute.

Previously, I tried using an attribute-only approach:

Tried using the updated_at attribute: this timestamp is updated if you update a Projelement‘s attributes, but is not updated if you update the embedded Comments. So I could manually update this_projelement.updated_at whenever a Comment is added. But the design feels brittle.

Also thought about having a separate attribute (e.g. last_updated_at) so that existing semantics of updated_at are not messed with. But design also feels brittle.

What would be the Rails way?

Answer 1

With MongoDB, there are no solution to order by some embedded document field.

In your case, the solution to do a callback before_save in your Comment and update an attribute in your parent document seem the most reasonnable technic.